The waves crashed loudly on the collapsed ruins of the Paseo del Atlántico, a walkway that once partially protected residents here from the volatile ocean. Erosion along this northernmost coast of Puerto Rico, nearly 20 miles east of San Juan, precipitated the promenade’s destruction, it finally fell into the Atlantic, exposing the Parcelas Suárez neighborhood to the water’s edge.

River deltas rank among the most economically and ecologically valuable environments on Earth. People living on deltas are increasingly vulnerable to sea-level rise and coastal hazards such as major storms, extremely high tides, and tsunamis.

The Chinese paddlefish — one of the world’s largest freshwater fish — has officially been declared extinct after surviving some 150 million years. The giant species, which measured as long as 23 feet and weighed as much as 1,100 pounds, has been killed off by overfishing and dam construction.

It’s easy to see why millions of people flock to the beach every year. They are dynamic places. With each crashing wave and changing tide, billions of pieces of sand and rock are constantly rearranged. This is what nature intended. What it did not, some scientists say, are the buildings that tower over some of the world’s most popular beaches.